Nil-ductility transition temperature is the temperature at which the fracture mode of the steel changes from ductile to brittle. Above the NDT a piece of steel typically will stretch or deform 20% - 40% before fracture when loaded to its ultimate tensile strength. However, below the NDT the same piece of steel will fracture in a brittle manner like glass when only loaded to yield strength (about half of the ultimate strength). Once it begins to crack in this manner the crack will continue to propagate at the speed of sound. It will only stop when it runs out of steel, the load is released, or until the crack is interrupted by steel that is behaving in a ductile manner due to different qualities in the steel or different temperatures present.